Built for the production app developer, not the ML engineer in a notebook. JS/TS gets equal billing with Python.
Phoenix has serious ML-observability DNA from Arize and is excellent if you're an ML engineer who lives in Python notebooks. Spanlens is built for the app developer who shipped an LLM feature last week, with proxy-first install, JS/TS equal-class with Python, statistical A/B testing, and a fully MIT-licensed codebase (Phoenix is source-available under ELv2).
| Feature | Spanlens | Arize Phoenix |
|---|---|---|
| Optimized for app developers | Yes | Partial |
| Optimized for ML engineers / notebook users | Partial | Yes |
| JS/TS as first-class language | Yes | Partial |
| Python as first-class language | Yes | Yes |
| 1-line baseURL proxy swap | Yes | No |
| OpenInference / OTel SDK instrumentation | Partial | Yes |
| TypeScript SDK | Yes | Partial |
| Python SDK | Yes | Yes |
| Per-request log with full body | Yes | Yes |
| Cost tracking | Yes | Partial |
| Agent tracing (waterfall) | Yes | Yes |
| Critical Path on agent traces | Yes | No |
| 3σ anomaly detection on latency/cost | Yes | Partial |
| Versioned prompt library | Yes | Partial |
| Production A/B traffic split | Yes | No |
| Built-in Welch t-test on A/B | Yes | No |
| LLM-as-judge scoring | Yes | Yes |
| Human annotation queue | Yes | Yes |
| Judge to human correlation tracking | Yes | Partial |
| Datasets / golden test sets | Yes | Yes |
| Embedding projector / drift analysis | No | Yes |
| Model swap recommendations with $ savings | Yes | No |
| Per-model cost breakdown & budget alerts | Yes | Partial |
| Security scanning (API keys, PII, prompt injection) | Yes | Partial |
| OSI-approved open-source license | Yes | No |
| Docker Compose self-host | Yes | Yes |
| Managed cloud with a free tier | Yes | Yes |
Updated 2026-06-03. Scroll for the grouped view with notes below.
Phoenix comes from Arize, an ML-observability company. Its UX reflects that audience (eval studios, embedding projectors, drift charts). Spanlens is built for the dev who shipped an LLM feature to production last week and needs cost, latency, and quality answers fast.
Phoenix is Python-first and JS support is lighter. Spanlens TypeScript SDK and proxy approach treat Next.js, Hono, Bun, and Cloudflare Workers as first-class citizens, the same surface as Python.
Phoenix uses OpenInference SDKs that wrap your client. You touch every call site. Spanlens is a baseURL swap. Existing apps with hundreds of LLM calls get instrumented in one config change.
Spanlens ships under MIT, an OSI-approved license you can use, modify, and even run as a service. Phoenix is Elastic License 2.0, which is source-available but restricts offering it as a managed service. Both now have a free hosted tier, so the real difference is what the license lets you do with the code.
Spanlens proactively flags routes where a smaller model would match quality. Phoenix has rich analysis but doesn't recommend cost-tier swaps.
Spanlens highlights the longest dependency chain in agent traces automatically, the actual bottleneck. Phoenix renders waterfalls but doesn't compute critical path.
Last updated 2026-06-03 · Spot something inaccurate? Let us know.
We don't think every team should pick us. Here's where Arize Phoenix legitimately wins.
If you work in notebooks, care about embedding drift, and want UMAP projections of your prompt space, Phoenix's ML-engineer DNA is exactly right. Spanlens optimizes for the production app developer instead.
Phoenix is the reference implementation for the OpenInference spec. If your org has standardized on it, Phoenix is the natural choice. Spanlens supports OTLP ingest but Phoenix's OpenInference lineage is deeper.
Phoenix can be launched inside a notebook for ad-hoc trace exploration during development. Spanlens is a server you point your app at, a different ergonomic.
If your team plans to graduate to Arize's full ML platform, starting on Phoenix means a smooth upgrade path. Spanlens is a destination, not a stepping-stone.
Phoenix comes from Arize, an ML-observability company. Its UX reflects that audience (eval studios, embedding projectors, drift charts). Spanlens is built for the dev who shipped an LLM feature to production last week and needs cost, latency, and quality answers fast.
Phoenix is Python-first and JS support is lighter. Spanlens TypeScript SDK and proxy approach treat Next.js, Hono, Bun, and Cloudflare Workers as first-class citizens, the same surface as Python.
Phoenix uses OpenInference SDKs that wrap your client. You touch every call site. Spanlens is a baseURL swap. Existing apps with hundreds of LLM calls get instrumented in one config change.
Spanlens ships under MIT, an OSI-approved license you can use, modify, and even run as a service. Phoenix is Elastic License 2.0, which is source-available but restricts offering it as a managed service. Both now have a free hosted tier, so the real difference is what the license lets you do with the code.
Spanlens proactively flags routes where a smaller model would match quality. Phoenix has rich analysis but doesn't recommend cost-tier swaps.
Spanlens highlights the longest dependency chain in agent traces automatically, the actual bottleneck. Phoenix renders waterfalls but doesn't compute critical path.
If you work in notebooks, care about embedding drift, and want UMAP projections of your prompt space, Phoenix's ML-engineer DNA is exactly right. Spanlens optimizes for the production app developer instead.
Phoenix is the reference implementation for the OpenInference spec. If your org has standardized on it, Phoenix is the natural choice. Spanlens supports OTLP ingest but Phoenix's OpenInference lineage is deeper.
Phoenix can be launched inside a notebook for ad-hoc trace exploration during development. Spanlens is a server you point your app at, a different ergonomic.
If your team plans to graduate to Arize's full ML platform, starting on Phoenix means a smooth upgrade path. Spanlens is a destination, not a stepping-stone.
If you're an ML engineer with a Python-first workflow, Phoenix fits your hands. If you're shipping LLM features in a Next.js, FastAPI, or Hono app and want zero-friction install, try Spanlens.
Free tier · No credit card · Self-host with Docker